As we delve deeper into GURPS, it has been repeatedly expressed by my players that character creation for this system is "backwards." That is, it represents the opposite sequence of events from the character creation of AD&D. In AD&D you roll your stats and see what you got before you even begin to plan. Perhaps you have a certain class in mind and roll a few times to get something that is passable. In GURPS, like most point-buy systems, if you haven't come in with a strong idea as to what you want to play, you're never going to be able to complete anything.
What are the benefits to the character creation systems as they stand? WE have the classic forward system, wherein you generate stats before you have an idea, and the modern reverse system, wherein you already know what you want to play when you grab the dice in hand (or the pencil, if you're working on a point-buy system). Of course, there are some hybrids (such as Deadlands), but they fall more or less into the modern reverse category, allowing you to jigger scores and switch things round about.
Firstly, they stem from two different roleplaying philosophies. These are not necessarily incompatible, they simply represent different types of games. There was a long period where I was heavily disdainful of point-buy and build-as-you-may systems, though I always made room for a few notable exceptions (see: 7th Sea).
Representationally, rolling for stats in the classic system achieves a number of goals, namely:
1) not every character can be optimized
2) not every character is good at everything
3) some characters are not naturally good at anything
4) distribution of classes occurs in a more or less arbitrary way, with stat requirements causing certain classes to appear less often in any given setting
1. Not every character can be optimized
Related to 2) yet still distinct, point 1) is a method to defeat would-be munchkins. When there's nothing to juggle, there's no way to game the system.
2. Not every character is good at everything
Obviously, given their druthers, everyone would pick an all-18 AD&D character. These superhumans rarely exist in life, so they should be rarely represented in games. Because we cannot trust our own instincts to protect us from creating characters that are just the greatest we can make them (related to point 1) above), this method takes that off our backs.
3. Some characters are not naturally good at anything
And yet they can be some of the best and most interesting characters when someone actually bothers to play them. This method forces people to play a variety of real-world averages.
4. Distribution of classes
This is one of the most powerful arguments for roll first choose after. It prevents everyone from playing the "best" classes and obviates the need to "balance" classes against each other. It also explains in-world demographics, which is a laudable goal.
5. Secret Philosophical Goal: You don't choose who you are before you're born
I always subscribed to this one. Life's hard, and often arbitrary. Deal with it.
The modern system has the following benefits:
1) no one has to have a character they don't want to play
2) its possible to construct a sensible group before the game even begins by choosing like backgrounds
3) group dynamics can cover more bases instead of having 3 of one character type, because the dice said so
4) characters don't have to start as powerless "level ones"
1. No one has to have a character they don't want to play
Of course, this is a corollary to the classic system. People don't have to play what they don't desire. Not sure this is a good thing... though it can be, from time to time.
2. Sensible groups with hooks fit together
Getting a D&D party together is always the most railroad-y part of any game. This way, the "party" can start pre-formed.
3. Group dynamics
No more ten wizard parties, however fun they are. Options are always better than not-options for game theory. Of course, this may stray into munchkin territory.
4. Level one
Not every game is suited for having a weak 16-year-old intro. The GURPS game, for example, would be nearly unplayable if everyone had to be a wageslave or a student. D&D has a very specific milieux where starting at level one is highly desirable for character formation. Other settings may not.